Nitrogen fertilizer is one of the most expensive nutrients to supply. 50-70% of the applied nitrogen is lost from the plant-soil system and causes water pollution (Peoples, 1995, in: P. E. Bacon, Editor, Nitrogen Fertilizer in the Environment, Marcel Dekker, 565-606). Improving nitrogen utilization efficiency (“NUE”) is important to reduce the cost of crop production as well as environmental damage. Nitrogen remobilization is one of the key steps to improve NUE (Mickelson et al., 2003, J Exp Bot 54, 801-812; Masclaux-Daubresse et al., 2008, Plant Biol (Stuttg) 10 Suppl 1, 23-36).
When plants encounter nutrient deficiency, nitrogen can be recycled from older to younger leaves to sustain the growth of developing tissues. Nitrate remobilization occurs not only from leaf to leaf during the vegetative stage, but also from leaf to seeds during the reproductive stage. High nutrient demand during reproductive stage cannot be satisfied by Nitrogen uptake, and nitrogen recycled from senescent tissue plays an important role in sustaining grain production. Although several studies showed that nitrate remobilization was important to increase grain yield and withstand nitrogen deprivation, little was known about nitrate remobilization. Thus, it is important to find out how the stored nitrate is retrieved to withstand nitrogen deficiency and to sustain high nitrogen demand in the reproductive stage, thereby to regulate the growth of nitrogen use efficiency in a plant.